If you just need icons for the application bar buttons, then start with the ones that come with the SDK. You'll find them at "C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Icons\Dark\" and these are also the ones that Blend can add for you.

Loads and loads of icons that you can search online. Download what you want in vector formats. Most are creative commons licensed and/or very cheap to license. (I've never paid more than $1.99 for an icon there.)

1300+ icons thast are free to use in personal and commercial projects. They're also helpfully categorized to make it easy to find what you're after.

There are also other sources and you could always make your own or commission an icon if you needed something special.

As an extrs tip, it's usually best, as far as possible, to get all your icons for an app from the same source. This helps make it look like they all belong together. If you have small/subtle differences between icons (e.g. differing line weights or the radius on rounded corners) then some people will spot this and will think it looks bad. Remember, the little details matter.